U.S. patent number 8,765,615 [Application Number 12/816,292] was granted by the patent office on 2014-07-01 for quartz-based mems resonators and methods of fabricating same.
This patent grant is currently assigned to HRL Laboratories, LLC. The grantee listed for this patent is David T. Chang, Randall L. Kubena, Hung Nguyen, Frederic P. Stratton. Invention is credited to David T. Chang, Randall L. Kubena, Hung Nguyen, Frederic P. Stratton.
United States Patent |
8,765,615 |
Chang , et al. |
July 1, 2014 |
Quartz-based MEMS resonators and methods of fabricating same
Abstract
A quart resonator for use in lower frequency applications
(typically lower than the higher end of the UHF spectrum) where
relatively thick quartz members, having a thickness greater than
ten microns, are called for. A method for fabricating same
resonator includes providing a first quart substrate; thinning the
first quartz substrate to a desired thickness; forming a metallic
etch stop on a portion of a first major surface of the first quartz
substrate; adhesively attaching the first major surface of the
first quartz substrate with the metallic etch stop formed thereon
to a second quartz substrate using a temporary adhesive; etching a
via though the first quartz substrate to the etch stop; forming a
metal electrode on a second major surface of the first quartz
substrate, the metal electrode penetrating the via in the first
quartz substrate to make ohmic contact with the metallic etch stop;
bonding the metal electrode formed on the second major surface of
the first quartz substrate to a pad formed on a substrate bearing
oscillator drive circuitry to form a bond there between; and
dissolving the temporary adhesive to thereby release the second
quartz substrate from the substrate bearing oscillator drive
circuitry and a portion of the first quartz substrate bonded
thereto via the bond formed between the metal electrode formed on
the second major surface of the first quartz substrate to and the
pad formed on the substrate bearing oscillator drive circuitry.
Inventors: |
Chang; David T. (Calabasas,
CA), Stratton; Frederic P. (Beverly Hills, CA), Nguyen;
Hung (Los Angeles, CA), Kubena; Randall L. (Oak Park,
CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Chang; David T.
Stratton; Frederic P.
Nguyen; Hung
Kubena; Randall L. |
Calabasas
Beverly Hills
Los Angeles
Oak Park |
CA
CA
CA
CA |
US
US
US
US |
|
|
Assignee: |
HRL Laboratories, LLC (Malibu,
CA)
|
Family
ID: |
50982040 |
Appl.
No.: |
12/816,292 |
Filed: |
June 15, 2010 |
Current U.S.
Class: |
438/756; 438/700;
438/745 |
Current CPC
Class: |
H01L
21/31056 (20130101); H03H 3/0072 (20130101); B81C
1/00238 (20130101); H01L 21/31111 (20130101); B81B
2201/0271 (20130101); H03H 2009/02291 (20130101); B81C
2203/038 (20130101); B81C 2203/036 (20130101); B81C
2201/0194 (20130101) |
Current International
Class: |
H01L
21/302 (20060101); H01L 21/311 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ahmed; Shamim
Assistant Examiner: Gates; Bradford
Attorney, Agent or Firm: Ladas & Parry
Government Interests
STATEMENT REGARDING FEDERAL FUNDING
This invention was made under U.S. Government contract
2007-1095-726-000. The U.S. Government has certain rights in this
invention.
Claims
What is claimed is:
1. A method of fabricating a resonator comprising: providing a
first quartz substrate having a desired thickness; forming a
metallic etch stop on a portion of a first major surface of the
first quartz substrate; adhesively attaching the first major
surface of the first quartz substrate and the metallic etch stop
formed thereon to a second quartz substrate using a temporary
adhesive, the metallic etch stop being directly adhesively bonded
to said second quartz substrate using said temporary adhesive;
etching a via though the first quartz substrate to the etch stop;
forming a metal electrode on a second major surface of the first
quartz substrate, the metal electrode penetrating the via in the
first quartz substrate to make ohmic contact with the metallic etch
stop; bonding the metal electrode formed on the second major
surface of the first quartz substrate to a pad formed on a
substrate bearing oscillator drive circuitry to form a bond there
between; and dissolving the temporary adhesive to thereby release
the second quartz substrate from the substrate bearing oscillator
drive circuitry and a portion of the first quartz substrate bonded
thereto via the bond formed between the metal electrode formed on
the second major surface of the first quartz substrate to and the
pad formed on the substrate bearing oscillator drive circuitry.
2. The method of claim 1 wherein the metallic etch stop forms an
electrode on the first major surface of the first quartz
substrate.
3. The method of claim 2 wherein the electrode on the first major
surface of the first quartz substrate comprises layers of Cr and Au
with the Cr layer disposed closer to the first quartz
substrate.
4. The method of claim 1 wherein the step of etching the via though
the first quartz substrate to the etch stop comprises: (a) defining
a metallic mask on the second major surface of the first quartz
substrate, the metallic mask including at least a first opening
therein for said via and at least another opening therein for
defining a perimeter shape of a quartz portion of said resonator;
and (b) etching through said at least a first opening in the
metallic mask to thereby etch through the first quartz substrate to
said etch stop and through said at least a another opening in the
metallic mask to thereby etch through the first quartz substrate to
define the perimeter shape of the quartz portion of said
resonator.
5. The method of claim 4 wherein the etching step is performed
using either a saturated ammonium bifluoride solution or
hydrofluoric acid solution.
6. The method of claim 1 wherein a plurality of resonators are
formed simultaneously from a single first quartz substrate, the
first quartz substrate having a plurality of metallic etch stops,
each etch stop being each formed on a portion of the first major
surface of the first quartz substrate, said portion aligning with
each one of said resonators for each one of said metallic etch
stops, the etching step etching said vias in each of said
resonators to each said etch stop, the etching step further etching
second vias to said first quartz substrate, the second vias
surrounding each one of said plurality of resonators.
7. The method of claim 1 wherein a second metallic electrode is
formed on a second major surface of the first quartz substrate
opposing the metallic etch stop formed on the first major surface
of the first quartz substrate, the second metallic electrode and
the first mentioned metallic electrode being electrically isolated
and physically spaced from each other.
8. The method of claim 1 wherein the first and second quartz
substrates are crystalline quartz.
9. The method of claim 1 wherein the first and second quartz
substrates are crystalline quartz and share a common crystal
orientation.
10. A method of fabricating a resonator comprising: providing a
first quartz substrate; thinning the first quartz substrate to a
desired thickness; forming a metallic etch stop on a portion of a
first major surface of the first quartz substrate; adhesively
attaching the first major surface of the first quartz substrate
with the metallic etch stop formed thereon to a second quartz
substrate using a temporary adhesive, wherein the temporary
adhesive covers substantially all of an exposed surface of the
first major surface of the first quartz substrate and all of the
metallic etch stop formed thereon; etching a via though the first
quartz substrate to the etch stop; forming a metal electrode on a
second major surface of the first quartz substrate, the metal
electrode penetrating the via in the first quartz substrate to make
ohmic contact with the metallic etch stop; bonding the metal
electrode formed on the second major surface of the first quartz
substrate to a pad formed on a substrate bearing oscillator drive
circuitry to form a bond there between; and dissolving the
temporary adhesive to thereby release the second quartz substrate
from the substrate bearing oscillator drive circuitry and a portion
of the first quartz substrate bonded thereto via the bond formed
between the metal electrode formed on the second major surface of
the first quartz substrate to and the pad formed on the substrate
bearing oscillator drive circuitry.
11. The method of claim 10 wherein the temporary adhesive is a
petroleum-based wax.
12. The method of claim 10 wherein a second metallic electrode is
formed on a second major surface of the first quartz substrate
opposing the metallic etch stop formed on the first major surface
of the first quartz substrate, the second metallic electrode and
the first mentioned metallic electrode being electrically isolated
and physically spaced from each other.
13. The method of claim 10 wherein the etching step includes
etching the first mentioned via though the first quartz substrate
to the etch stop and also etching a second via spaced laterally
from the first mentioned via and spaced laterally from the metallic
etch stop, the second via surrounding the resonator.
14. The method of claim 10 wherein the second quartz substrate has
a planar region which is adhesively bonded to both the first quartz
substrate and to the metallic etch stop formed on the first quartz
substrate using said temporary adhesive which covers substantially
all of the exposed surface of the first major surface of the first
quartz substrate and all of the metallic etch stop formed
thereon.
15. A method of fabricating a resonator comprising: providing a
first quartz substrate; thinning the first quartz substrate to a
desired thickness; forming a metallic etch stop on a portion of a
first major surface of the first quartz substrate; adhesively
attaching the first major surface of the first quartz substrate and
the metallic etch stop formed thereon to a second quartz substrate
using a temporary adhesive, the temporary adhesive covering
substantially all of an exposed surface of the first major surface
of the first quartz substrate and all of the metallic etch stop
formed thereon; after adhesively attaching the first major surface
of the first quartz substrate and the metallic etch stop formed
thereon to the second quartz substrate, simultaneously etching a
first via though the first quartz substrate to the etch stop and a
second via though the first quartz substrate to the temporary
adhesive; forming a metal electrode on a second major surface of
the first quartz substrate, the metal electrode penetrating the via
in the first quartz substrate to make ohmic contact with the
metallic etch stop; bonding the metal electrode formed on the
second major surface of the first quartz substrate to a pad formed
on a substrate bearing oscillator drive circuitry to form a bond
there between; and dissolving the temporary adhesive to thereby
release the second quartz substrate from the substrate bearing
oscillator drive circuitry and a portion of the first quartz
substrate bonded thereto via the bond formed between the metal
electrode formed on the second major surface of the first quartz
substrate to and the pad formed on the substrate bearing oscillator
drive circuitry.
16. The method of claim 15 wherein the temporary adhesive is a
petroleum-based wax.
17. The method of claim 15 wherein the metallic etch stop forms an
electrode on the first major surface of the first quartz
substrate.
18. The method of claim 17 wherein the electrode on the first major
surface of the first quartz substrate comprises layers of Cr and Au
with the Cr layer disposed closer to the first quartz
substrate.
19. The method of claim 15 wherein the step of etching the via
though the first quartz substrate to the etch stop comprises: (a)
defining a metallic mask on the second major surface of the first
quartz substrate, the metallic mask including at least a first
opening therein for said via and at least another opening therein
for defining a perimeter shape of a quartz portion of said
resonator; and (b) etching through said at least a first opening in
the metallic mask to thereby etch through the first quartz
substrate to said etch stop and through said at least a another
opening in the metallic mask to thereby etch through the first
quartz substrate to define the perimeter shape of the quartz
portion of said resonator.
20. The method of claim 19 wherein the etching step is performed
using either a saturated ammonium bifluoride solution or
hydrofluoric acid solution.
21. The method of claim 15 wherein a plurality of resonators are
formed simultaneously from a single first quartz substrate, the
first quart substrate having a plurality of metallic etch stops,
each etch stop being each formed on a portion of the first major
surface of the first quartz substrate, said portion aligning with
each one of said resonators for each one of said metallic etch
stops.
22. The method of claim 15 wherein a second metallic electrode is
formed on a second major surface of the first quartz substrate
opposing the metallic etch stop formed on the first major surface
of the first quartz substrate, the second metallic electrode and
the first mentioned metallic electrode being electrically isolated
and physically spaced from each other.
23. The method of claim 15 wherein the first and second quartz
substrates are crystalline quartz.
24. The method of claim 15 wherein the first and second quartz
substrates are crystalline quartz and share a common crystal
orientation.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
None
TECHNICAL FIELD
This invention relates to quartz-based Micro ElectroMechanical
Systems (MEMS) resonators and methods of fabricating same,
particularly for resonators having relatively thick quartz members
for operation at frequencies below approximately
BACKGROUND
The technology disclosed in U.S. Pat. No. 7,237,315 issued Jul. 3,
2007 and entitled "Method for Fabricating a Resonator" is improved
upon by this invention. Both the prior art and this invention use
similar MEMS fabrication technology to form a quartz resonator
structure. However, the technology disclosed in U.S. Pat. No.
7,237,315 really works best when used to make resonators which
operate at frequencies at the upper end of the UHF band or even
higher.
But there is a need for quartz resonators which operate at even
lower frequencies (less than 50 MHz, for example). A problem arises
when using the technology of U.S. Pat. No. 7,237,315 to try to make
lower frequency resonators--the thickness of the quartz resonator
must be increased, but due to the vastly different quartz
thicknesses between the higher end of the UHF band in one hand and
lower frequency devices on the other hand (several microns of
quartz thickness for the upper UHF frequency devices compared with
several tens or hundreds of microns thickness for lower frequency
devices), the soft photoresist mask used in U.S. Pat. No. 7,237,315
cannot be successfully utilized. In U.S. Pat. No. 7,237,315 the
photoresist is the "soft" mask which is used with plasma dry
etching. In this disclosure a "hard" mask is used instead because
the presently disclosed method uses a wet etchant at an elevated
temperature for typically many hours. A "soft" mask can not
withstand such an aggressive wet etch and therefore a Cr/Au (metal
hard) mask is suggested herein. The soft masks can be seen used in
FIGS. 1f and 1h of the prior art which are used for etching the via
and the resonator.
This invention also introduces a novel quartz resonator temporary
attachment and release technology that can increase device yield
and lower cost. In U.S. Pat. No. 7,237,315 silicon or GaAs were
used as the handle wafer because the quartz resonator wafer was
bonded to the handle using a room temperature direct bond without
any adhesives. The handle can be dissolved or etched away later
using a preferential etch that does not attack quartz. As long as
you can find a material that can be directly bonded to quartz at
room temperature and preferentially removed later, you can use
it.
In this disclosure, a quartz handle is suggested, which is
inconsistent with the prior art because with the prior art direct
bonding process the quartz handle can not be preferentially removed
without also attacking to the quartz resonator wafer.
Also not having to form the cavity in the handle as done in the
prior art is an improvement for the quartz handle in that putting a
cavity into a quartz substrate can be omitted. The adhesive bond to
the handle is a high temperature bond that occurs at 150.degree. C.
(for example).
The process flow from the above-identified US patent is shown, in
simplified form, by FIGS. 1a-1k. The process begins by providing a
quartz substrate 2 having a first surface 3 and a second surface 5,
a silicon or GaAs handle substrate 4, and a base or host substrate
14. A portion of the silicon handle substrate 4 is etched away
creating a cavity 6, as shown in FIG. 1b. The etched cavity 6 can
be fabricated with a wet etch of potassium hydroxide, or a dry
reactive ion etch using a gas having a fluorine chemistry. Then,
top-side electrode and tuning pad metal (Al or Au) 7 is deposited
onto a quartz substrate 2 as shown by FIG. 1c. Next, the two wafers
2, 4 are brought together using a direct bonding process as
depicted by FIG. 1d. After a low temperature bonding/annealing
operation, a combination of processes including wafer
grinding/lapping, chemical-mechanical-planarization (CMP), plasma
etching and chemical polishing is used to thin the quartz down to a
thickness, typically less than 10 microns, for a desired resonant
frequency as depicted by FIG. 1e. Next, photolithography is used to
pattern contact via holes in the thinned quartz layer 2. The holes
are etched through quartz to stop on top-side electrode metal 7 and
then metallized to form the through-wafer metal vias 11 as shown in
FIG. 1f. The bottom-side electrodes 12 are then metallized (see
FIG. 1g), and the quartz layer is patterned and etched (see FIG.
1h) to form an array of resonators. Finally, protrusions are etched
into the host substrate 14, and metalization patterns 16, including
bonding pads, are defined on the substrate 14 as depicted by FIG.
1i. The quartz/silicon pair 2,4 is bonded to the host wafer 14
using either a Au--Au or Au--In compression bonding scheme (see
FIG. 1j), and the silicon handle wafer 4 is thereafter removed with
a combination of dry and wet etches, resulting in the quartz
resonators being attached only to the host wafer, as shown in FIG.
1k. The prior art uses a spin coating of a soft mask (photoresist)
for patterning of the metal, quartz and silicon structures.
BRIEF DESCRIPTION OF THE INVENTION
In one aspect the present invention provides a method for
fabricating a resonator, the steps of which include: providing a
first quart substrate; thinning the first quartz substrate to a
desired thickness; forming a metallic etch stop on a portion of a
first major surface of the first quartz substrate; adhesively
attaching the first major surface of the first quartz substrate
with the metallic etch stop formed thereon to a second quartz
substrate using a temporary adhesive; etching a via though the
first quartz substrate to the etch stop; forming a metal electrode
on a second major surface of the first quartz substrate, the metal
electrode penetrating the via in the first quartz substrate to make
ohmic contact with the metallic etch stop; bonding the metal
electrode formed on the second major surface of the first quartz
substrate to a pad formed on a substrate bearing oscillator drive
circuitry to form a bond there between; and dissolving the
temporary adhesive to thereby release the second quartz substrate
from the substrate bearing oscillator drive circuitry and a portion
of the first quartz substrate bonded thereto via the bond formed
between the metal electrode formed on the second major surface of
the first quartz substrate to and the pad formed on the substrate
bearing oscillator drive circuitry.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a-1l depict a known process for making a UHF and higher
frequency resonator with a quartz substrate.
FIG. 2a depicts the starting materials: a quartz device wafer, a
quartz handle wafer, and oscillator ASIC wafer;
FIG. 2b shows a top side electrode/etch stop metalization formed on
the quartz device wafer;
FIG. 2c depicts a temporary adhesive applied to quartz handle
wafer;
FIG. 2d depicts the quartz device wafer temporally bonded to quartz
handle wafer;
FIG. 2e shows the effect of grinding and polishing quartz device
wafer a desired thickness;
FIG. 2f depicts the metalization formed and patterned using
photolithographic techniques as wet etch mask for both vias and
resonators;
FIG. 2g shows the effect of wet etching quartz vias and resonators
for both vias and resonators;
FIG. 2h depicts metalization deposited and patterned for via metal
and for bond pads;
FIG. 2i depicts metalization deposited and patterned for a bottom
side electrode;
FIG. 2j depicts a wafer of quartz resonators being bonded (as full
wafer bonding, but only one device is shown in this figure) to a
wafer having a similar plurality oscillator ASICs formed hereon in
appropriate positions to mate with the plurality of quartz
resonators temporarily held by the quartz handle;
FIG. 2k depicts the temporary adhesive being dissolved to free
quartz handle wafer and allow its removal; and
FIG. 2l depicts one of a plurality of released quartz resonators on
ASIC wafer.
FIG. 3 depicts a plan view of the etch mask formed by an upper
layer of metalization (also shown in elevational view by FIG. 2g)
having an opening therein for etching a via through the quartz
resonator to an underlying etch stop formed as discussed with
reference to FIG. 2b.
DETAILED DESCRIPTION
The resonant frequency of a thickness-shear mode quartz resonator
is inversely proportional to the quartz layer thickness. Both this
invention and the prior art listed describe thickness-shear mode
devices, the former operating at HF-UHF bands, and the latter at
VHF-UHF bands.
A preferred embodiment of the process flow for quartz resonator
fabrication according to the present invention is illustrated in
FIGS. 2a-2l. As depicted by FIG. 2a, the starting materials
preferably consist of a quartz device wafer 20 (for the resonator),
a quartz handle wafer 22, and a host substrate 24 (such as a
silicon CMOS ASIC wafer with oscillator drive circuitry disposed
thereon). A quartz handle wafer 22 is chosen instead of a silicon
handle used in the prior art because it offers better thermal
matching to the quartz device wafer 20. In order to obtain the best
thermal match between the quartz handle wafer 22 the quartz
resonator wafer 20, one should preferably use wafers of the same
starting crystal orientation because quartz is an anisotropic
material that has two different coefficients of thermal expansion
(CTE--which occur parallel or perpendicular to the z axis of the
crystal). For example, a Z-cut quartz resonator wafer 20 should
preferably be bonded to a Z-cut quartz handle wafer 22 and an
AT-cut (35 degrees rotated from z-axis) quartz resonator wafer 20
should preferably be matched with an AT-cut quartz handle wafer
22.
Thermally (CTE) matched wafers 20, 22 are used for the handle wafer
22 and for the quartz device wafer 20, otherwise, the bonded pair
20, 22 would either warp severely or break due to the stress caused
by thermal expansion mismatch between the quartz resonator wafer
and the Ga or Si handle. A silicon-quartz or GaAs-quartz bonded
pair as used in U.S. Pat. No. 7,237,315 will likely fail during a
hot ammonium bifluoride wet etch, for example, due to the thermal
expansion mismatch between such materials.
Fabrication begins with forming a top-side metalization 26 on a
portion of the quartz device wafer 20 preferably using conventional
fabrication techniques known to those skilled in the art. See FIG.
2b. This metalization 26 is preferably formed as a metal stack
(e.g., layers of Cr and Au with the Cr closest to the quartz device
layer 20) and is used to form the top electrode of the resonator
and also to act as an etch stop for a subsequent etch.
Turning to FIG. 2c, a temporary adhesive 28, either a
petroleum-based wax or a high temperature epoxy, is coated or
otherwise applied onto the handle wafer 22. The quartz device wafer
20 is then bonded to the handle 22 using the previously applied
adhesive 28 to form a thermo-compression bond there between as
shown by FIG. 2d. A preferred adhesive 28 is a petroleum-based wax
sold under the trade name Apiezon W by SPI Supplies/Structure
Probe, Inc. of West Chester, Pa. 19381. This wax can be dissolved
using tetrachloroethylene when time comes to free the completed
resonator from the handle wafer 22 (discussed below with reference
to FIG. 2k).
The quartz device wafer 20 is subsequently thinned to a desired
thickness (see FIG. 2e) to reflect the operating frequency
preferably using both wafer grinding and chemical mechanical
planarization (CMP) techniques to reduce the thickness of the
quartz device wafer 20 to a desired thickness. The resulting
thickness of the quartz device wafer 20 ranges from ten microns to
hundreds of microns.
Next, a metalization layer 30, such as Cr/Au (preferably comprising
one or more layers of Au on one or more layers of Cr with a Au
layer preferably comprising a final exposed layer of the Cr/Au
sandwich), is deposited and etched to form a via openings 31 and
33. The resonator mask 30 also depicted by FIG. 3. The mask 30 is
used for wet etching of the thinned quartz device wafer 20. A
through-wafer etch of the thinned quartz device layer 20 is
preferably performed in either a saturated ammonium bifluoride
solution or hydrofluoric acid. The wet etchant stops etching via 31
when the wet etchant reaches metalization 26 (which also serves as
an etch stop layer 26 and as the resonator's top electrode as
discussed above with reference to FIG. 2b). The metalization/etch
stop 26 only acts as an etch stop for only one of the three vias
depicted (via 31). The other via (via 33) surrounds the quartz
resonator arm 35 allowing it to be released from neighboring
resonator arms (not shown) when the adhesive 28 is dissolved in a
subsequent step. The etching of vias 31 and 33 preferably occur
simultaneously.
In FIG. 3 three dashed box outlines 34, 38-1 and 38-2 are also
depicted. These outlines depict the placement of elements formed
during subsequent manufacturing steps. Outline 34 shows where the
metalization 34 for resonator's bottom electrode will be preferably
located (see also FIG. 2i and the related discussion below).
Outline 38-1 shows where pad 38 will eventually align with
metalization 32 for forming a thermo-compression bond to electrode
26 (see FIG. 2j and the related discussion below). Outline 38-2
shows where another pad on ASIC 24 will eventually align with a
metalization 34.2 which is coupled to (and formed with)
metalization 34 by means of a metalization connector 34.1 (shown by
a dashed line in FIG. 3). Metalization 34.1 and 34.2 provide for
ohmic contact between the bottom electrode 34 and circuitry of the
ASIC 24.
Turning to FIG. 2h, the etch mask formed by metalization 30 is now
stripped and a new layer of metalization 32, preferably formed of a
stack of Cr/Au (preferably comprising one or more layers of Au on
one or more layers of Cr with Au comprising the last exposed
layer), is conformally deposited within via 31 and on the resonator
arm 35 and etched to form a resonator bond pad and via metal 32 to
connect to metalization 26 (which will become the top electrode
after the device is inverted). The resonator bond pad shown in this
view is the flat portion of metalization 32 which mates with the
ASIC on wafer 24 as depicted by FIG. 2j. Another metalization layer
34, preferably also of a Cr/Au stack, is deposited and patterned to
form what will become the bottom electrode 34 of the quartz
resonator, as well metalization 34.1 and 34.2 shown in FIG. 3.
Metalization 32 and 34, 34. and 34.2 are preferably formed together
(that is, at the same time and therefore may have the same
thickness, unless metalization 32 has more layers of Cr/Au than
does metalization 34). Metalization 26 and 34, 34.1 and 34.2
preferably have the same thicknesses.
On the host substrate, ASIC 24, probe pads 36 (preferably
comprising Cr/Pt/Au layers with Au comprising the last exposed
layer) and substrate bond pads 38 for thermo-compression bonding
(preferably comprising Cr/Pt/Au/In layers) are deposited. See FIGS.
2a and 2j, for example, which show these pads 36 and 38 (see also
FIG. 3 which depicts where the substrate bond pads are preferably
positioned by outlines 38.1 and 38.2).
The ASIC substrate 24 probe pads 36 and ASIC substrate 24 bond pads
38 are typically not fabricated at a silicon CMOS foundry where the
remaining portions of the ASIC 24 are conventionally fabricated, so
they are typically added as a post CMOS processing operation by the
fabricator of the quartz resonator. The addition of these pads 36
and 38 to ASIC 24 is preferably done separately from the quartz
fabrication of the resonator 35 itself, but these pads 36 and 38
need to be added before the quartz device/handle pair 20, 22 is
aligned and bonded to ASIC 24 as is described below.
Only one substrate bond pad 38 is depicted in FIGS. 2a and 2j, and
that pad 38 is used to ohmically connect metalization 26 to the
circuitry of the ASIC via the pad 38 and metalization 32 depicted
in FIG. 2j. It is to be understood, however, that a second
substrate bond pad for thermo-compression bonding (preferably
comprising Cr/Pt/Au/In layers) is also fabricated on ASIC 24, which
second pad is used to ohmically connect metalization 34 to the
circuitry of the ASIC via that a second substrate bond pad. The
second substrate bond pad is not depicted in FIGS. 2a and 2j, since
it is preferably located behind the pad 38 shown in these views,
but spaced therefrom so as to be ohmically isolated from the pad 38
seen in these figures. The location of the second substrate bond
pad is depicted by outline 38.2 of FIG. 3, however,
Although not shown in FIGS. 2i-2l for ease of illustration,
metalization 34 preferably extends behind metalization 32 to make
contact with the aforementioned second substrate bond pad (via
compression bonding) when substrate bond pad 38 is compression
bonded to metalization 32. The metalization extending behind
metalization 32 is shown as a dashed line 34.1 in FIG. 3 and the
pad which mates with the aforementioned second substrate bond pad
(outline 38-2) is shown as a dashed box 34.2 in FIG. 3.
As shown by FIG. 2j, the quartz device/handle pair 20, 22 is then
aligned and bonded to the host ASIC wafer 24 with a metal-metal
(preferably Au to In) thermo-compression bond at the two substrate
bond pads mentioned above, preferably using a commercial wafer
bonder. The bonded stack can be soaked in an appropriate solvent to
dissolve the adhesive 28, thus freeing the resonator 25 from the
handle wafer 22 as shown in FIG. 2k. Since a plurality of
resonators 25 are typically formed at one time (only one is
completely shown in FIGS. 2a-2l as being formed for ease of
illustration and explanation, but a two dimensional array of
resonators 25 would preferably be formed at one time from a common
sheet of quartz 20), a plurality of resonators 25 are released when
a common quartz handle 22 is released by the aforementioned
solvent. Dissolving the adhesive 28 with a solvent as opposed to
etching the quartz handle away as done in the prior art is another
advantage because the process is both quick and safe (it does not
attack the completed quartz resonators).
The fully released devices (see FIG. 2l) are then preferably baked
in a vacuum oven to completely rid themselves of any residual
solvent and then preferably diced into individual resonators.
Having described the invention in connection with certain
embodiments thereof, modification will now suggest itself to those
skilled in the art. As such, the invention is not to be limited to
the disclosed embodiment except as is specifically required by the
appended claims.
* * * * *